Researchers from the consumer magazine Which found that high alkaline, or pH levels, in the water was reducing the effectiveness of the disinfectant to just 10% of its maximum potential.

“This could put swimmers at a high risk of catching infection,” says a report in the latest edition of the magazine.

Which tested 61 pools across the country and identified Dorking as one of three falling in the top “potentially serious” category.

Mole Valley Council, which owns the pool, maintains that the water is completely safe. However, it says that testing procedures are being reviewed to ensure any risk is minimised.

During their survey Which representatives turned up at pools unannounced and asked permission to take water samples. These were tested at the side of the pools to assess the chemical control of the water. The samples were then sent to a laboratory for analysis for specific bacteria.

Of the 61 samples, 35 fell outside the acceptable standards for water quality. Nine posed a health risk to bathers and three, including Dorking, posed a potentially serious risk.

Says the report: “In most cases this was because of low or ineffective disinfectant levels in the water which could allow bacteria to multiply and cause infections.”

The Dorking pool is managed by a private company, Leisure Connection, on behalf of the council. The council says that the pool is tested every two hours during opening times.

In addition, regular bacteriological tests are carried out by environmental health inspectors. The council claims these have shown that the pool is free of harmful bacteria.

Of the Which report, the council says it draws attention to the “theoretical potential risk to health” which Leisure Connection and the council takes very seriously.

“The testing procedures are already very vigorous but are being reviewed to ensure that any risk is minimised,” it adds.

The council says an independent consultant has confirmed that the pool’s pH is high. But in a statement it adds: “The natural chemical used to stabilise the pH will give an artificial picture and will rectify itself in time.

“As all other procedures are in line with current recommendations, and no bacteria is present, this apparently high reading will only be a temporary situation.”

The pool, at Reigate Road, is due to close once the new Dorking Sports Centre opposite, with its 25m pool, opens next spring. It too will be managed by Leisure Connection.

Sue Threader, Mole Valley’s director of social services, said state-of-the-art equipment was being installed at the new pool.

“In the meantime, we would like to reassure customers of the swimming centre that the pool is completely safe and that all necessary steps will be taken to keep it safe,” she added.

The pool was closed this week while maintenance work was being carried out in the plant room. Officials said this was unconnected with the Which report and that the pool would reopen as soon as possible, perhaps by the weekend.